// run: $exec < input
#include <iostream>
#include <iomanip>

int const maxn = 55;
double fact[maxn];
double f[maxn][maxn];
int a[maxn];
int n, p;

int main()
{
	std::cin >> n;
	fact[0] = 1;
	for (int i = 1; i <= n; i++) {
		std::cin >> a[i];
		fact[i] = fact[i - 1] * i;
	}
	std::cin >> p;
	f[0][0] = 1;
	for (int i = 1; i <= n; i++)
		for (int j = n; j >= 1; j--)
			for (int k = p; k >= a[i]; k--)
				f[j][k] += f[j - 1][k - a[i]];

	double ans = 0;
	for (int i = 1; i <= n; i++)
		for (int k = 1; k <= p; k++)
			ans += f[i][k] * fact[i] * fact[n - i];
	std::cout << std::fixed << std::setprecision(8) << ans / fact[n] << '\n';
}

